In the purification of particulate matter (PM) contained in diesel exhaust gas, a diesel particulate filter (DPF) has been typically used. PM originates mainly in fuel, and is produced from organic material, which is an easily combustible component, soot, which is a difficulty combustible component, and the like. The combustion temperature of organic components is roughly 200° C. to 550° C.; however, temperatures of roughly 550° C. to 700° C. are required in soot combustion. As a result, a conventional catalyzed soot filter (CSF) made by supporting a noble metal catalyst on a DPF shows high purification performance for organic materials, but the purification performance for soot is low.
In view of such facts, the present situation is such that, during PM purification, soot combustion is forcibly performed using additional technology such as regeneration and additives. However, PM collection by a DPF becomes a cause of exhaust gas pressure loss, and by carrying out forced regeneration, troubles occur in that there are disadvantages of the DPF from loss in fuel efficiency and PM combustion heat, and catalyst failure. Therefore, lowering the combustion temperature of PM containing soot has been attempted in order to reduce the burden on automobiles.
A method and the like has been considered in which PM is continuously combusted by a catalyst, in addition to a reduction in regeneration frequency and a high efficiency additive technology, as a method for reducing the burden on automobiles. A noble metal catalyst, a composite oxide catalyst, etc. have been proposed as PM low temperature combustion catalysts used in such a method. According to these catalysts, although acknowledged to be effective in the combustion of soot, the combustion temperature is still a high temperature of 450° C. to 600° C.
For example, a catalyst composition free of platinum group metals has been proposed that includes 5 to 90 mol % by Ag equivalent of ceria (CeO2) stabilized by Ag and/or Co (refer to Patent Document 1). The catalyst composition is prepared by a nitrate decomposition method in this invention, and according to a DPF provided with such a catalyst composition, even if at low temperatures, it is active compared to a DPF equipped with a conventional catalyst, and it is said to excel in both being economical and efficient.
In addition, a DPF cleaning apparatus provided with a catalyst containing silver, copper and oxides thereof and ceria has been proposed (refer to Patent Document 2). According to this invention, a filter used in filtered removal of PM is made that can efficiently purify over the entire operating range of a diesel engine.
In addition, utilization of a composite oxide containing Ag2O as a catalytically active compound has been proposed in a method that oxidizes liquid or solid pollutants in a gas stream (refer to Patent Document 3). In this invention, the composite oxide containing Ag2O is manufactured by a solid phase reaction method, and according to this method, it is made possible to reduce and prevent the release of pollutants by separating liquid or solid pollutants from the exhaust gas, and combusting.
In addition, a filter for purifying diesel particulates that includes a heat-resistant 3-dimensional structure that is porous, an intermediate layer composed of LiAlO3 supported on a surface of the heat-resistant 3-dimensional structure, and a catalyst layer supported on an upper surface of the intermediate layer, and uses a catalyst made by supporting a noble metal on a composite oxide, has been proposed (refer to Patent Document 4). According to this invention, it is made possible to prevent emission of SOF by efficiently adsorbing and oxidatively combusting SOF in a low temperature range, while being able to combust dry-soot by utilizing the combustion heat of SOF.
In addition, since nitrogen dioxide (NO2) exhibits high activity in PM combustion, a method of arranging an NO2-generating catalyst upstream of the DPF (refer to Patent Document 5) and a method of applying an NO2-generating catalyst to the DPF (refer to Patent Document 6) have been proposed. According to these methods, even if in a low temperature range in which PM itself normally does not combust, it is made possible to combustively remove PM by utilizing NO2. However, under conditions in which there is little NOx, an effect that promotes combustion of PM is insignificantly small, and under high temperature conditions, the effect of promoting PM combustion by NO2 becomes very small since the concentration equilibrium of NO and NO2 favors the NO side.
In addition, a technique of combining an NO2-generating catalyst and a PM combustion catalyst is also well known since PM combustion efficiency is further increased by NO2. For example, an exhaust purification apparatus of a diesel engine has been proposed that includes a filter in which a first catalyst for NO2 generation and a second catalyst on which NO2 thus converted is reacted with PM are supported in two layers or in a mixed layer state (refer to Patent Document 7). According to this invention, it is made possible to efficiently remove PM by causing NO2 generated by the first catalyst and PM to react at the second catalyst. However, in this invention, since the second catalyst is coated on the first catalyst, or coated in a mixed state, the contact probability of PM with the second catalyst is actually unusually low, and the effect may not be sufficiently exhibited.
In addition an exhaust gas purification material has been proposed that is provided with coated layers including a noble metal catalyst layer in which a noble metal is supported on an inorganic oxide in an outer layer, and including a transition metal catalyst layer containing a transition metal in an inner layer (refer to Patent Document 8). According to this invention, high catalytic activity to PM combustion is obtained. However, since PM combustion is a solid-solid reaction, it is possible for the outer layer catalyst and PM to contact; however, the inner layer catalyst and PM cannot contact, and thus the performance of the inner layer catalyst cannot be sufficiently exhibited.    Patent Document 1: Japanese Patent Application, Publication No. 2004-42021    Patent Document 2: Japanese Patent Application, Publication No. 2001-73748    Patent Document 3: Japanese Translation of PCT International Publication, Publication No. 2000-502598    Patent Document 4: Japanese Patent Application, Publication No. H08-173770    Patent Document 5: Japanese Patent Application, Publication No. H01-318715    Patent Document 6: Japanese Patent Application, Publication No. 2003-293730    Patent Document 7: Japanese Patent Application, Publication No. 2001-263051    Patent Document 8: Japanese Patent Application, Publication No. 2001-157845